A system having a signal receiving function such as a radar or communication system is designed to improve reception sensitivity by reducing the noise temperature of the reception subsystem. The noise temperature of this subsystem is generally caused by the transmission loss occurring in the transmission line from the antenna to a low-noise amplifier (LNA) and the internal noise generated in the LNA.
As a method of reducing this transmission loss and internal noise, there has been proposed a method of housing the transmission line from the antenna to the LNA, electronic circuits such as a reception filter, and the LNA in a thermally insulating container such as a vacuum vessel and cooling them to a superconducting phase by using a refrigerating unit or the like. Cooling the transmission line and the electronic circuits to a superconducting phase will reduce the transmission loss occurring in the transmission line from the antenna to the LNA close to zero. In addition, cooling the LNA to a superconducting phase will reduce the internal noise in the LNA. The conventional method proposed above is designed to improve the sensitivity of a reception subsystem by reducing the transmission loss between the antenna and the LNA to almost zero and reducing the internal noise in the LNA by using a superconductivity technique.
A conventional antenna apparatus based on the above proposal uses an airtight coaxial connector for an electrical (high-frequency) interface which connects a normal-temperature unit outside a thermally insulating container to a cryogenic unit inside the thermally insulating container. The antenna apparatus uses a coaxial cable in the thermally insulating container for connection from the coaxial connector to the high-frequency circuit.
The antenna apparatus having the above arrangement, however, uses the coaxial connector and the coaxial cable for the coupling portion between the normal-temperature unit and the cryogenic unit, and hence the normal-temperature unit is connected to the cryogenic unit through the outer conductors (metal) of the coaxial connector and coaxial cable. In this case, heat from the normal-temperature unit flows into the cryogenic unit through the outer conductors (metal), raising the temperature of the cryogenic unit. It is therefore necessary to provide a cooling capacity that takes into account the rise in temperature due to heat flow. This inevitably increases the size of a cooling means such as a refrigerating unit.
As a method of suppressing an increase in the size of the above cooling means, it is conceivable to use a method of reducing heat flow by decreasing the diameter of the coaxial cable. This method, however, increases the transmission loss in the coaxial cable, and hence the reception sensitivity deteriorates. In contrast, if the diameter of the coaxial cable is increased to maintain high sensitivity, the heat flow increases. In this case, since it is necessary to consider an increase in temperature due to the increased heat flow, the size of the cooling means increases. In addition, the above connection method requires the use of an expensive airtight coaxial connector. It is also necessary to structurally ensure a size that allows the connector to be mounted.